Referring to FIG. 1, an example of the optical monitoring technique in the wavelength division multiplex transmission (WDM) system will be explained. In a node in which an optical monitoring system is installed, some wavelength of an optical signal S3 included in a WDM optical signal S1 sent out from an upstream node is demultiplexed by an optical demultiplexer 102, and is dropped off through a transponder 106. Moreover, an optical signal S4 that is to be added in this node is added to an optical signal S2 that is included in the optical signal S1 except what was dropped therefrom through a transponder 108, becoming an optical signal S6 that is multiplexed in an optical multiplexer 104. The optical signal S6 is amplified by an optical amplifier 110, a part thereof is extracted by an optical branching device 112 as an optical signal S5 for monitoring, and the part except the extracted optical signal is made to transmit to a downstream side. An optical monitoring module 120 monitors the optical signal S5. The optical monitoring module 120 detects a peak value of signal light, and measures various parameters.
For monitoring an optical signal, an optical monitoring functional device for detecting and monitoring a signal of a bandwidth that is equivalent to a transmission speed of about 10 Gbit/sec can be used. In the case of such a transmission speed, even if the wavelength resolution of an optical monitoring module inside an optical monitoring functional device was coarse, the optical signal could be detected without error.
Listed below are documents relevant to optical monitoring techniques of the optical communication system.    1. Japanese Laid-Open Patent Application JP-P2006-189465A    2. Japanese Laid-Open Patent Application JP-P2006-074698A    3. Japanese Laid-Open Paten Application JP-A-Heisei, 9-252283